1. Field of the Invention
The present invention relates to a method of fabricating graphene quantum dots, and more particularly, relates to a method of fabricating graphene quantum dots of light-emitting properties using metallic hydrate salts, and also related to high quality graphene quantum dots fabricated by the method.
2. Description of the Related Art
Quantum dot is a crystal structure less than several tens of nanometers having light-emitting property. The quantum dot can emit light of various wavelengths by controlling the band gap energy of the quantum dot.
If the size of a material reduced to nanoscale, the energy level wherein electrons can exist is quantized. For example, the quantum confinement effect which does not appear in common materials appears in nanoparticles of several nanometers.
Since the band gap energy of a semiconductor nanoparticle relatively increases as the size of a semiconductor nanoparticle decreases, the semiconductor nanoparticle can emit dozens of colors only by controlling slightly the size of the semiconductor nanoparticle even the same material. Thus, the nanoparticles of semiconductor materials smaller than 10 nm or smaller than Debroglie wavelength have great potentials for applications such as electronic products for displays, recording devices, various sensors, nanocomputers, biological and medical applications, and so on.
The band gap between a conduction band and a valence band can be controlled using CdSe, CdTe, CdS, etc. which are commonly used for materials of quantum dots. The emitting wavelength can be controlled by controlling the size of quantum dots when an electron transit from higher level to lower level. Smaller quantum dot emits blue or purple light of short wavelength and larger quantum dot emits red light of long wavelength. Thus, the quantum dot can emit all colors of the light by controlling the size of it.
However, Cd(2+) precursors and Se(2−) precursors and capping with organic materials are necessary for fabricating the CdSe quantum dots. The fabrication method of CdSe quantum dots has environmental and economic problems because trioctylphosphine (TOP) for a reaction source is highly poisonous and expensive. The process instability of fabricating the CdSe quantum dots prevents the mass production because the reaction temperature is carried out at a high temperature above 300° C.
Graphene-based quantum dots can overcome the above mentioned problems and a display device using the graphene-based quantum dots can reduce leakage current by more than 20% compared with that of CdSe quantum dots which are commonly used for the display device.
A lithography method or a chemical synthesis method is used for fabricating quantum dots. The chemical synthesis method needs relatively more simple apparatus than the lithography method for fabricating quantum dots.
The resolution of lithography strongly depends on a lithography apparatus. A quantum dot of several to several tens of nanometers can be fabricated using an extra ultraviolet (EUV) lithography apparatus, an e-beam apparatus, or an x-ray apparatus, but the production cost is high. It is also difficult to produce quantum dots with uniformed shape and size on the whole substrate using a common lithography apparatus.
The method for fabricating quantum dots using a deposition method such as physical vapor deposition or chemical vapor dispersion also has limitation in producing and controlling quantum dots with uniformed distribution and size.
Chemical reduction method is synthesizing graphene with flakes of graphene oxide exfoliated from graphite. Forming the graphene solution enables low temperature firing and mass synthesis. The dispersion stability of the graphene is excellent, but the graphene has high sheet resistance.
In order to apply quantum dots of graphene to a display apparatus, high quality graphene with a single layer is necessary. Therefore, a fabrication method of controlling the size of the graphene quantum dot is required for natural colors.
Korean published patent No. 10-2011-0106625 discloses a method of fabricating 2-dimensional nanostructured graphene by forming intercalation compound of graphite after inserting alkali metal ions or alkaline earth metal ions between layers of the graphite.
In addition, as conventional methods of oxidation/reduction process, a heat treatment method in a autoclave after oxidation process of the graphene with sulfuric acid or nitric acid (Dengyu Pan et al., “Hydrothermal Route for Cutting Graphene Sheets into Blue-Luminescent Graphene Quantum Dots”, Adv. Mater. 2010, 22, 734-738), a microwave treatment method (Ling-Ling Li et al., “A Facile Microwave Avenue to Electrochemiluminescent Two-Color Graphene Quantum Dots”, Adv. Funct. Mater. 2971-2979, 22, 2012), a hydrazine (N2H4) liquid treatment method (Sung Hwan Jin et al., “Tuning the Photoluminescence of Graphene Quantum Dots through the Charge Transfer Effect of Functional Groups”, Acs. Nano. 1239-1245, 7, 2013) were reported.
However, the conventional method for fabricating the graphene quantum dots requires severely oxidation/reduction processes of the graphite. Complicate reaction process, expensive fabrication costs and low yield are yet remained as problems to be solved.